An existing brushless motor driving circuit uses a switching drive circuit as shown in FIG. 8. Such a switching drive circuit for a brushless motor in general causes an electrical switching noise and a mechanical noise such as vibration of a coil caused by the switching noise, because of the following reasons.
Upon on-off actions of transistors Tr1 to Tr6 responsive to a switching signal, a driving voltage equal to the switching signal is applied to coils Lu to Lw. Due to application of the voltage to the coils, a transient response causes a high frequency component of the switching noise to ride on a current I which flows in the coils. Further, rotations of the motor causes a counter electromotive voltage in the coils, and due to changes in the counter electromotive voltage a high frequency component of a further complicated noise is superposed on the current in the coils.
The current in the coils is detected by a current detector Rs. Since the current waveform includes a lot of above-indicated noises, high frequency components of the noise are also superposed on an output of a current feedback amplifier A.sub.2.
Therefore the high frequency components of the noises are also superposed on the switching signal (analog signal) whose amplitude is changed by the output signal of the current feedback amplifier A.sub.2.
This means that a high frequency component (noise) is produced in the coils and amplified to a signal having a tremulously serrated waveform as shown in FIG. 9.
Since the superposed high frequency component (noise) on the current in the coils gives a bad influence to peripheral circuits, the noise must be removed.
In this regard, the driving circuit of FIG. 8 uses filters consisting of electrolytic capacitors Ccu, Ccv and Ccw and resistors Ru, Rv and Rw and connected to the driving coils Lu, Lv, Lw so as to remove such noises at the terminal end of a control loop.
This countermeasure, however, involves a drawback that a large through current I, which flows in the capacitors upon high-speed rotations of the motor, possibly destroys the capacitors, and this disables the use of the existing driving circuit for a high speed of 2000 rpm or more.
Further, when such electrolytic capacitors are used, the manufacturing cost is increased, and a considerable mounting space is occupied.